Light transmission through a liquid crystal display (LCD) is non-linear when viewed from a central position with respect to the plane of the display. For example, as illustrated in FIG. 1, an LCD screen for a portable computer or a projection TV will normally have better transmittance and color rendition near the center of the display when viewed head-on than at the upper and lower edges, since the viewing angle is somewhat different for each scan line from top to bottom on the screen when viewed directly at an angle of 0.degree. with the center line. The transmittance is a function of the applied voltage and is defined in the art by a V-T or .gamma. - curve. As the transmittance is variable with the angle .THETA. at which a scan line of the display is viewed, a different light transmittance or .gamma. - curve exists for each different viewing angle of the display, i.e., a curve exists for head-on viewing at 0.degree., for upward viewing at +10.degree., and when viewing downward at -10.degree., as shown in FIG. 2. As a practical matter, one .gamma. - curve is selected for correcting purposes, and in the conventional method for correcting the .gamma. - curve of an LCD, the correction curve at 0.degree. is used and is typically applied by means of a voltage divider resistance network such that the correction curve is fixed. Consequently, the best .gamma. - curve correction is typically not obtained, and poor color resolution results, in view of the fact that the head and tail of the curve are not straight so that the darkest and whitest spots cannot be seen.
Other attempts to deal with the problem of poor resolution at the edges have focused on the liquid crystal materials and the changing of their characteristics. There is normally a significant difference in the transmittance characteristics of the three LCD scan lines at -10.degree., 0.degree., and +10.degree. viewing angle so that corrective approaches try to change their characteristics to cause overlap. This approach has been found to be very difficult to accomplish so that a satisfactory solution has not been found.
Another approach is directed to modifying the control circuitry for applying voltages to the LCD. One example of a prior art disclosure dealing with circuitry for gamma correction is found in U.S. Pat. No. 5,461,430 to J. G. Hagerman wherein a gamma correction circuit is directed to correcting the grey scale linearity of images displayed on a liquid crystal light valve and cathode ray tube combination using a plurality of amplifiers, each adapted to implement a predetermined transfer function, and configured to compensate for nonlinearity in the display. The gamma correction here is used in conjunction with dynamic threshold correction and involves the inclusion of current sources with the amplifiers.
It will therefore be seen from a consideration of the prior art that various approaches have been used to deal with the problem in the art of achieving a method and means that will improve .gamma. - curve correction in LCDs so as to achieve optimum transmittance and color resolution to a viewer over the face of the display.
It is accordingly an object of the present invention to provide a method and means for improving .gamma. - curve correction in LCDs whereby the transmittance and color resolution are optimized over the face of the display screen for a viewer observing the screen head on.
It is a further object of the present invention to provide a method and means for improving .gamma. - curve correction in LCDs using a non-material approach wherein correction is achieved by calculating a suitable correction curve and implementing its application with appropriate circuitry.